Quaternary Organic Amines Inhibit Na,K Pump Current in a Voltage-dependent Manner : Direct Evidence of an Extracellular Access Channel in the Na,K-ATPase

The effects of organic quaternary amines, tetraethylammonium (TEA) chloride and benzyltriethylammonium (BTEA) chloride, on Na,K pump current were examined in rat cardiac myocytes superfused in extracellular Na⁺-free solutions and whole-cell voltage-clamped with patch electrodes containing a high Na⁺-salt solution. Extracellular application of these quaternary amines competitively inhibited extracellular K⁺ (K⁺_o) activation of Na,K pump current; however, the concentration for half maximal inhibition of Na,K pump current at 0 mV (K⁰_Q) by BTEA, 4.0 ± 0.3 mM, was much lower than the K⁰_Q for TEA, 26.6 ± 0.7 mM. Even so, the fraction of the membrane electric field dissipated during K⁺_o activation of Na,K pump current (λ_K), 39 ± 1%, was similar to λ_K determined in the presence of TEA (37 ± 2%) and BTEA (35 ± 2%), an indication that the membrane potential (V_M) dependence for K⁺_o activation of the Na,K pump current was unaffected by TEA and BTEA. TEA was found to inhibit the Na,K pump current in a V_M-independent manner, i.e., inhibition of current dissipated 4 ± 2% of the membrane electric field. In contrast, BTEA dissipated 40 ± 5% of the membrane electric field during inhibition of Na,K pump current. Thus, BTEA inhibition of the Na,K-ATPase is V_M-dependent. The competitive nature of inhibition as well as the similar fractions of the membrane electric field dissipated during K⁺_o-dependent activation and BTEA-dependent inhibition of Na,K pump current suggest that BTEA inhibits the Na,K-ATPase at or very near the enzyme's K⁺_o binding site(s) located in the membrane electric field. Given previous findings that organic quaternary amines are not occluded by the Na,K-ATPase, these data clearly demonstrate that an ion channel–like structure provides access to K⁺_o binding sites in the enzyme.